We hypothesized that vestibular otolith organ stimulation contributes to human orthostatic responses. Twelve subjects underwent three 60° upright tilts: (1) with the neck flexed from 0° to 30° relative to the body during 60° tilt, such that the head moved from horizontal to 90° above horizontal (0 to 1 Gz otolith stimulation); (2) with the head and body aligned, such that they tilted together to 60° (0 to 0.87 Gz otolith stimulation); and (3) with the neck flexed 30° relative to the body during supine conditions, and the neck then extended to -30° during 60° body tilting, such that the head remained at 30° above horizontal throughout body tilting (constant 0.5 Gz otolith stimulation). All three tilt procedures increased thoracic impedance, sympathetic nerve activity (N=8 of 12), arterial pressure, and heart rate relative to supine conditions (all P<0.04). Within the first 20 s of tilt, arterial pressure increased most obviously in the 0 to 1 Gz otolith condition. Thoracic impedance tended to increase more in otolith-constant conditions, but no dependent variable differed significantly between tilt conditions, and no significant time×tilt interactions emerged. Otolith inputs may contribute to early transient adjustments to orthostasis. However, lack of significant main effects of tilt condition and time×tilt interactions suggests that potential otolith effects on the variables we studied are relatively subtle and ephemeral, or that other mechanisms compensate for a lack of change in otolith input with orthostasis.
- Head-up tilt
- Heart rate
- Muscle sympathetic nerve activity